1. Field of the Invention
The present invention relates to sensing circuitry and a sensing method for detecting a change in voltage on at least one input line to the sensing circuitry.
2. Description of the Prior Art
Sensing circuitry is often used in integrated circuits, and may be used in a variety of implementations, for example in semiconductor memories, microprocessors, large logic arrays, etc. Such sensing circuitry has voltage change detection circuitry for detecting a change in voltage on at least one input line and for producing at least one output signal indicative of that change. The voltage change detection circuitry typically comprises one or more latch transistors used to produce at least one output signal indicative of the detected change in voltage.
Whilst such latch transistors can take a variety of forms, one known approach involves using one or more latch transistors that have a body region insulated from a substrate. The body region comprises the channel material in which a channel is established between the source and drain of the transistor below the gate of the transistor. By using latch transistors having a body region insulated from the substrate, it has been found that this reduces the capacitive loading on the transistor terminals when compared with latch transistors formed from non-insulated technologies where the channel material is formed on a conducting substrate. This reduced capacitive loading can give rise to increase switching speed of the transistors and lower power dissipation.
One known technology that can be used to produce transistors having a body region insulated from the substrate is referred to as Silicon-On-Insulator (SOI) technology, where the SOI channel material is formed within a region of a thin superficial silicon layer above an oxide insulating layer and located under the gate of the transistor, reducing resistive leakage to the substrate and capacitive loading by the substrate. Consequently, this body region is not biased to any known voltage, and its voltage can vary depending on charges arising from diode leakage, coupling capacitance with the gate, drain or source, impact ionisation, etc. Additionally, the voltage on the body region becomes dependent on the previous circuit activity, which is typically referred to as the “history effect”. This variation in voltage on the body region can have a significant impact on the behaviour of a circuit constructed using such transistors, since any change in the body voltage will typically modify the threshold voltage of the transistor, thus modifying the current passing through the transistor and the switching speed of the transistor. When such transistors are used in sensing circuitry used to detect a change in voltage on at least one input line, such changes in the body voltage can give rise to incorrect operation of the sensing circuitry.
One known technique used to remove the earlier-mentioned history effect involves biasing the bodies of certain transistors within the sensing circuitry to a fixed reference voltage, for example by coupling the bodies to ground for an N-channel transistor, or coupling the bodies to the power supply voltage for a P-channel transistor. Whilst this can remove the history effect, it is disadvantageous in that this body biasing can give rise to an increase in MOSFET (metal oxide semiconductor field effect transistor) threshold voltage, and so can give rise to delayed switching, thus increasing the time taken by the sensing circuitry to detect a change in voltage.
U.S. Pat. No. 6,433,589 describes an alternative approach to biasing the transistor bodies with a fixed reference. In particular, a sense amplifier is described where the bodies of amplifier transistors within a sense amplifier and bodies of input transistors to the sense amplifier are coupled to corresponding input signals, thereby eliminating the history dependence that would result from unconnected bodies, while achieving faster switching times due to a dynamically produced difference in threshold voltage of the input transistors and amplifier transistors. However, whilst such an approach may improve switching times when compared with circuits using input transistors and amplifier transistors having statically biased bodies, such an approach can give rise to a number of disadvantages.
Firstly, by making a direct connection between the bodies of the transistors and the input signals, these input signals being the bit line signals in the circuit described in U.S. Pat. No. 6,433,589, this involves applying the supply voltage or a voltage close to the supply voltage to the body of the transistors. Consequently, the diode formed between the body region and the source region of such transistors will become strongly forward-biased as soon as the source of the transistor is lowered to ground potential, this occurring every time the sense amplifier is turned on to perform a sensing operation. Additionally, the diode will remain strongly forward-biased for as long as the voltage on the source is maintained at the ground potential, and hence will remain strongly forward-biased for as long as the sense enable signal is active, i.e. during the whole sensing operation. This can give rise to significant extra power consumption due to the forward-biased diode current, and can give rise to disturbance of the connected bit line signal, thereby producing noise.
In addition, the presence of the forward-biased diode causes the connected bit lines to discharge more than would otherwise be the case, thus resulting in further increased power consumption required after the sensing operation has completed in order to raise the voltage on those bit lines back to the source potential VDD.
US-A-2005/0264324 describes systems and methods for increasing the amount of current that can flow through the data line pull-down transistors in a sense amplifier by tying the bodies of those transistors to a voltage other than ground. In one embodiment, the bodies of data line pull-down transistors are tied to the intermediate nodes on the opposing side of the sense amplifier to increase the current flow through the data line pull-down transistors, which increases the speed at which the sense amplifier can be operated. Further, because the voltages at the intermediate nodes change, the threshold voltages of the transistors also change, thereby enhancing the operation of the sense amplifier. However, since one of the intermediate nodes will remain at the source voltage level during the sensing operation, and that source potential is applied to the body of one of the transistors on the other side of the sense amplifier, this will again give rise to a forward-biased diode in that transistor, thus giving rise to additional power consumption. This forward-biased diode can also give rise to noise on an associated bit line assuming that the bit line remains connected to the sense amplifier during the sensing operation.
Accordingly, it would be desirable to provide an improved sensing circuitry and method of operation of such circuitry which is able to maintain robustness to the history effect described earlier, but without the disadvantages associated with the above-mentioned prior art schemes.